Composite amplifier, a radio terminal and a method for improving the efficiency of the composite amplifier

ABSTRACT

The present invention relates to a composite amplifier (3, 4, 120), a radio terminal (100) including such composite amplifier and to a method for improving the efficiency of such composite amplifier in particular. The composite amplifier according to embodiments of the present invention is arranged to be connected to an output combiner network (43, 53, 63, 73, 83) and to a load (49, 130). The output combiner network comprising at least one dynamically tuneable reactance (47, 48). The instantaneous efficiency of the composite amplifier (3, 4, 120) is increased by tuning the impedances/admittances seen by each of said at least two power amplifiers (41, 42, 71, 72). The amplifiers being differently driven and they may further be part of a Chireix outphasing system or a pair of a Doherty amplifier.

TECHNICAL FIELD

The present invention relates to power amplifiers in general and to acomposite amplifier, a radio terminal including such composite amplifierand to a method for improving the efficiency of such composite amplifierin particular.

BACKGROUND

Power amplifiers usually used in radio transmitters for broadcast,cellular, and satellite systems, are indispensable components that haveto be efficient and linear, in addition to being able to simultaneouslyamplify many radio channels (frequencies) or independent user datachannels, spread across a fairly wide bandwidth. A power amplifier, suchas a radio frequency (RF) power amplifier, also has to performamplifications efficiently in order to reduce power consumption and toincrease its longevity. High linearity is required because a non-linearinput-output signal characteristic of a power amplifier often results ina broadened spectrum around the desired amplified signal, and anunwanted in-band component of the signal, which lead to bad systemperformance especially in multicarrier telecommunications systems (e.g.WCDMA) which are known to be particularly sensitive to the effects ofnon-linearities.

To decrease the effects of non-linearity, several linearization schemescould be used. One such linearization scheme is known as feed-forward,where a signal is injected after the amplifier that cancels thenon-idealities. Another linearization scheme usually used is topredistort (modify) the signal at the input of the amplifier in order togive an undistorted amplified signal at the output of the amplifier.This technique is called predistortion.

An additional important key factor of RF power amplifiers used inmulticarrier telecommunications systems (e.g. WCDMA) is as mentionedabove, the amplifier efficiency.

The amplifier efficiency must be kept high in order to reduce the needfor cooling as well as the overall power consumption, and to increasethe lifetime of the power amplifier. Conventional power amplifiers havelow efficiency especially when transmitting signals with a largepeak-to-average power ratio. As an example, a wideband signal generallyhas an average power (amplitude) that is normally much smaller than thepeak power (amplitude) and since a conventional linear RF poweramplifier generally has an efficiency that is proportional to its outputamplitude, its average efficiency is consequently very low for suchsignals having a large peak-to-average power ratio.

In response to the low efficiency of conventional linear poweramplifiers when transmitting signals with a large peak-to-average powerratio, two methods or two amplifier structures have been widelyutilized. The Doherty amplifier (or the Doherty amplification method),is described in W. H. Doherty, “A new high efficiency power amplifierfor modulated waves,” Proc. IRE, vol. 24, no. 9, pp. 1163-1182,September 1936, and the Chireix outphasing system (or the Chireixamplification method) is described in H. Chireix, “High power outphasingmodulation”, Proc. IRE, vol. 23, no. 11, pp. 1370-392, November 1935.

The Doherty amplifier uses one non-linear and one linear amplifier. Afirst power amplifier is driven as a linear amplifier in class B, and asecond power amplifier having non-linear output current modulates theimpedance seen by the first amplifier, through an impedance-invertingquarter wave line. Since the non-linear output current of the secondpower amplifier is zero below a certain output voltage, the second poweramplifier does not contribute to power loss below this voltage.

The Doherty amplifier's output power level back-off where the efficiencyreaches a maximum in the efficiency curve of the Doherty amplifier is athalf the maximum output voltage. The location of the output power levelback-off can be changed by changing the impedance of the quarter-wavetransmission line (or transformation (matching) network). Thus the sizeof the transformation (matching) dictates the location of the lowerefficiency maximum of the Doherty power amplifier. Even though theDoherty amplifier can be extended to three or more amplifiers in orderto obtain more maximum points on the efficiency curve, this usuallyleads to a requirement for very unevenly sized amplifiers i.e.transistors.

The principal of the Chireix outphasing system is to create amplitudemodulation using two amplifiers operating at constant amplitude togetherwith a special type of combining network. By altering the differentialphase-shift between the two amplifiers, amplitude modulation is created.The combination of generally two phase modulated constant amplitudesignals thus enables amplitude modulation. After up-conversion andamplification through RF chains (e.g. mixers, filters and amplifiers),the signals are combined to form an amplified signal in an outputcombiner network. The phases of these constant amplitude signals arechosen so that the result from their vector-summation yields the desiredamplitude. The compensation reactances, denoted +jX and −jXrespectively, in the output network of the Chireix amplifier, are usedto extend the region of high efficiency to include lower output powerlevels. The efficiency of Chireix systems is derived in R. F. Raab,“Efficiency of outphasing RF power amplifier systems”, IEEE Trans.Communications, vol. COM-33, no. 10, pp. 1094-1099. October 5.

An advantage with the Chireix amplifier is the ability to change theefficiency curve to suit different peak-to-average ratios. The peakoutput power is equally divided between the amplifiers irrespective ofthis adjustment, which means that equal size amplifiers can be used. Thechange of the efficiency curve may be performed by changing (tuning) thesize of the reactances (X) in order to tune a combining network of aChireix amplifier, thus achieving peak efficiency at an average outputpower. This approach is proposed in M. El-Asmar, A. B. Kouki “ImprovingChireix Combiner Efficiency Using MEMS Switches”, IEEE CCECE/CCGEI,Ottawa, pages 2310-2313, May 2006.

In the above mentioned prior art publication, the length of tuning stubsthat are used to create the compensation reactances for the Chireixcombiner, is varied. The response time of MEMS (Micro Electro MechanicalSystem) switches are used to connect and disconnect the different stubs(usually two) at the input of the combiner. This exchange of the twostubs occurs at a fixed level of the phase between the two input signalsin order to increase the efficiency of the Chireix power amplifier.MEMS-switches are however mechanical devices which means reliabilityissues over time. Furthermore, the switches available today are commonlyquite small and thus will be severely affected by the amount of powerpassed through the combiner network from each amplifier of the Chireixamplifier. In addition, finite switch time may cause additional problemsby introducing “jumps” by the load seen from each amplifier thusaffecting the efficiency of the amplifier.

In general, the Chireix and Doherty amplifiers have efficiency maxima atsome fixed medium output levels. This is considered optimal for somefixed signal amplitude distribution but less than optimal for all other.This is due to that the efficiency getting lower moving away from thesesignal envelope amplitudes.

In the U.S. Pat. No. 7,221,219, a composite power amplifier structure issuggested which essentially is based on a combination of the auxiliaryamplifier of a Doherty amplifier and at least one pair of amplifiersforming a Chireix pair. The Doherty part of the composite amplifier isdriven in the same manner as the auxiliary amplifier of a Dohertyamplifier. Each Chireix pair is driven by drive signals having amplitudedependent phase over at least a part of the dynamic range of thecomposite amplifier. In this prior art document, the efficiency of thecomposite amplifier is improved by letting the different pairs haveamplitude dependent phase in different part of the dynamic range of thecomposite amplifier.

Another method of improving the average efficiency of a RF poweramplifier is by dynamically adjusting a matching network of the RF poweramplifier. However, the dynamic matching components of the matchingnetwork may be slow, may lose efficiency above those of fixed componentsand/or may require significant power in order to perform the adjustmentsince the power is generally proportional to the bandwidth of theadjustment/tuning process.

SUMMARY

An object of the present invention is thus to obviate at least some ofthe above disadvantages by providing a procedure for improving theinstantaneous efficiency of a composite amplifier. The compositeamplifier according to the present invention comprises at least twopower amplifiers which are differently driven and which may beconfigured to be a pair of a Chireix outphasing system or a pair of aDoherty amplifier.

According to a first aspect of the invention, the above stated problemis solved by means of an apparatus in terms of a composite amplifier foruse in a radio terminal of a telecommunications system, the compositeamplifier comprising at least two power amplifiers that are differentlydriven and that are further arranged to be connected to an outputcombiner network and to a load, the output combiner network comprisingat least one dynamically tuneable reactance. In order to increase theinstantaneous efficiency of the composite amplifier in accordance withthe present invention, at least one of the impedances seen by each ofthe at least two differently driven power amplifiers are configured tobe dynamically tuned.

According to another aspect of the present invention, the above statedproblem is solved by means of a method for improving the instantaneousefficiency of a composite amplifier comprising at least one dynamicallytuneable reactance and further comprising at least two differentlydriven power amplifiers being connected to an output combiner networkand to a load. The method comprising: dynamically tuning at least one ofthe impedances seen by each of the at least two differently driven poweramplifiers such that the instantaneous efficiency of the compositeamplifier is increased.

According to yet another aspect of the present invention, the abovestated problem is solved by means of a radio terminal comprising acomposite amplifier according to the above mentioned compositeamplifier.

An advantage of the present invention is that the efficiency degradationdue to voltage overhead associated with a dynamic matching amplifierwith slow tuning components can be mitigated since the use of slowdynamic matching components requires the allocation of headroom for thesignal to account for quick excursions of the amplitude. Thus, aseparate allocation of headroom is not necessary in a compositeamplifier according to the present invention.

Another advantage of the present invention is that the instantaneousefficiency of the composite amplifier that is comprised of amplifierswhich may be configured to be a pair of a Chireix outphasing system or apair of a Doherty amplifier, is improved since a Chireix amplifier or aDoherty amplifier has an efficiency maximum some bit below the peakoutput amplitude point, meaning that the efficiency of the compositeamplifier can be maximized over a longer time than that of the envelopeamplitude fluctuations of the output signal and still have amplitudeheadroom for quick upward excursions. Thus, in accordance with thepresent invention, the composite amplifier in accordance with thepresent invention enables decoupling the efficiency maximum from theamplitude maximum.

Yet another advantage of the present invention is that reliabilityperformance due to non-mechanical operations is increased.

A further advantage of the present invention compared to e.g. thepreviously described MEM-solution, is that the use of dynamicallytuneable reactances in accordance with the present invention, providesan improvement in the instantaneous efficiency of the compositeamplifier compared to the use of MEMS-switches. Although moreMEMS-switches may provide high efficiency, they also increase thereliability risks contrary to solutions provided by the presentinvention.

The present invention will now be described in more details by means ofseveral embodiments and with reference to the accompanying drawings,attention to be called to the fact, however, that the following drawingsare illustrative only, and that various modifications and changes may bemade in the specific embodiments illustrated as described within thescope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram illustrating the principles of a prior artDoherty amplifier.

FIG. 1B is a schematic diagram illustrating the efficiency of a priorart Doherty amplifier.

FIG. 2A is a block diagram illustrating the principles of a prior artChireix outphasing system.

FIG. 2B is a schematic diagram illustrating the efficiency of a priorart Chireix outphasing system without compensating reactances.

FIG. 3A is a block diagram illustrating a general structure of anexemplary composite amplifier in accordance with an exemplary embodimentof the present invention.

FIG. 3B is a block diagram illustrating the general structure of FIG. 3Aincluding the impedances/admittances seen by the amplifiers.

FIGS. 4A-4D schematically illustrate examples of the efficiency of acomposite amplifier including a Chireix outphasing system in accordancewith embodiments of the present invention and further illustratecorresponding Smith diagrams.

FIG. 5A illustrates an output voltage as a function of a scaling factorfor different constant outphasing angles.

FIG. 5B illustrates an example of the instantaneous efficiency curves ofa composite amplifier according to embodiments of the present invention.

FIGS. 6A-6B illustrates different structures of a composite amplifierincorporating a Chireix outphasing system, in accordance with otherembodiments of the present invention.

FIGS. 7A-7B illustrates different structures of a composite amplifierincorporating a Doherty amplifier, in accordance with other embodimentsof the present invention.

FIG. 8 illustrates the efficiency of a composite amplifier incorporatingof a Doherty amplifier.

FIG. 9 is a block diagram illustrating a radio terminal including acomposite amplifier according to exemplary embodiments of the presentinvention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, scenarios, techniques, etc. in order to provide thoroughunderstanding of the present invention. However, it will be apparentfrom the person skilled in the art that the present invention and itsembodiments may be practiced in other embodiments that depart from thesespecific details.

The present invention is described herein by way of reference toparticular example scenarios. In particular the invention is describedin a non-limiting general context in relation to an exemplary WCDMAsingle carrier signal having a predetermined peak-to-average power ratio(PAPR) (e.g. 7 dB PAPR). Note again that the present invention and itsexemplary embodiments are by no means restricted to the above mentionedWCDMA single carrier signal i.e. multicarrier or single carrier signalsof other type of systems may be used. In addition, the present inventionis described in a non-limiting general context in relation to acomposite amplifier comprising at least two power amplifiers which maybe configured to be a pair of a Doherty amplifier or a pair of a Chireixoutphasing system.

Referring to FIG. 1A there is illustrated a block diagram of a typicalprior art Doherty amplifier 1. A Doherty amplifier consists of twoamplifiers called the main amplifier 10 and the peak amplifier 11. Themain and peak amplifiers 10, 11 are illustrated as equally large, eventhough they do not have to be of equal size. As illustrated in FIG. 1A,the two amplifiers are connected by a quarter-wave transmission linewith characteristic impedance Z_(o). The output of the peak amplifier isadditionally connected to a load R_(L). It is here assumed that bothamplifiers 10 and 11 act as ideal controlled generators, i.e. the outputcurrents (i₁ and i₂) are proportional to an input drive signal. If wenow assume that the optimal load resistance of one of the amplifiers isR_(opt), then, the load resistance should equal R_(opt)/2, so that themaximum deliverable power is twice that of a single amplifier. The roleof the transmission line is to transform the load resistance to 2R_(opt)at the output of the main amplifier. If we assume that the outputimpedance of the main amplifier is infinite, the impedance seen by thepeak amplifier 11 will be zero due to the impedance invertingcharacteristics of the transmission line. At a low output level, thepeak amplifier is completely turned off and presents infinite outputimpedance. This means that the RF voltage on the main amplifier (v₁)rises twice as fast when we increase the current (i₁) as for aconventional amplifier (the load resistance is 2R_(opt) as compared toR_(opt)), giving about twice the efficiency. At some point, called thetransition point, this voltage has reached its maximum, with acorresponding maximum in efficiency. When saturation is reached, theDoherty amplifier 1 starts driving current from the peak amplifier 11,which is transformed through the transmission line to a voltage on themain amplifier 10. By selecting the phase of i₂ to lag 90 degrees behindthat of i₁, the voltage contribution to v₁ from the peak amplifier 11will be 180 degrees out of phase from the contribution from the mainamplifier 10. This means that the voltage v₁ remains constant as wegradually increase i₁ and i₂. Since the load resistance seen by the peakamplifier 11 is zero, the voltage v₂ is not affected by i₂, but willinstead equal R_(opt)i₁. The resulting efficiency of a prior art Dohertyamplifier is illustrated in FIG. 1B.

Referring to FIG. 2A there is illustrated a block diagram of a typicalprior art Chireix outphasing system 2. The term outphasing, which is thekey method in Chireix and LINC (linear amplification with non-linearcomponents) amplifiers, generally means the method of obtainingamplitude modulation by combining two phase-modulated constant-amplitudesignals produced in a signal component separator 22. After up-conversionand amplification through RF chains 24, 26 (e.g. mixers, filters,amplifiers) and power amplifiers 28 and 30, the outphased signals arecombined to form an amplified linear signal in a Chireix type outputcombiner network 32. The phases of these constant amplitude outphasedsignals are chosen so that the result from their vector-summation yieldsthe desired amplitude. The Output combiner network 32 includes twoquarter-wave lines λ/4 (where λ represents the wavelength of the centerfrequency of the frequency band at which the amplifier is operated) andtwo compensating reactances, denoted +jX and −jX, which are used toextend the region of high efficiency to include lower output powerlevels. Also illustrated in FIG. 2A is a load R_(LOAD) which in FIG. 2Arepresents an antenna. The efficiency of a prior art Chireix outphasingsystem without compensating reactances is illustrated in FIG. 2B.

Generally, power amplifiers are modeled as comprising a matching or acombiner network that is characterized by at least oneimpedance/admittance seen from one of several power amplifiers and/or byat least one transimpedance from one of several power amplifiers to anoutput node.

As an example, the combiner network of a Doherty amplifier may becharacterized by the impedance seen by the main amplifier as a functionof both the characteristic impedance of the transmission line and theload impedance R_(L) that is connected to the combiner network. For theChireix outphasing system, the combiner network may be characterized bythe quarter-wave transmission lines together with the reactances +jX and−jX. As an example, the impedance/admittance seen by one of theamplifiers of the Chireix outphasing system may be expressed as afunction of the reactance +jX or −jX and the load impedance R_(L) thatis connected to the combiner network.

In the following, exemplary embodiments of a composite amplifieraccording to the present invention are presented. The exemplarycomposite amplifier may comprise at least two power amplifiers which maybe configured to be a pair of a Doherty amplifier or a pair of a Chireixoutphasing system. In both cases, the combiner network of the compositeamplifier comprises at least one dynamically tuneable reactance and thepower amplifiers are configured to be differently driven. Theinstantaneous efficiency of the composite amplifier is, according toembodiments of the present invention, increased by dynamically tuning atleast one of the impedances seen by each of the power amplifiers.

FIG. 3A illustrates a general schematic view of a composite amplifier 3according to an exemplary embodiment of the present invention. Thecomposite amplifier of FIG. 3A is here considered to comprise a Chireixoutphasing system including two differently driven amplifiers 41 and 42that are connected to a combiner network 43. The Combiner network 43 isrepresented by two quarter-wave transmission lines 45 and 46 havingcharacteristic impedances Z₀. The combiner network 43 further comprisestwo dynamically tuneable reactances (−jX) 47 and (+jX) 48 that arecontrolled. The combiner network 33 is also connected to a load (R_(L))49. According to an embodiment of the present invention, the impedanceZ₁ seen by e.g. power amplifier 41 and the impedance Z₂ seen byamplifier 42 may be dynamically tuned in order to increase theinstantaneous efficiency of the composite amplifier 3. Thetuning/retuning may be performed by dynamically tuning/retuningreactances 47 and 48 i.e. by dynamically tuning/retuning −jX and +jX.Note here that in the case of a Chireix outphasing system, thedynamically tuneable reactances −jX and +jX (or similarly theimpedance/admittance seen by each of the amplifiers) are a function ofan outphasing angle denoted here φ.

As is well known in the art, an admittance Y can be defined by Y=G+jB,where G is a conductance and B is a susceptance. The admittance can alsobe written as the inverse of the impedance Z i.e.:

$\begin{matrix}{Y = {\frac{1}{Z} = \frac{1}{R + {j\; X}}}} & (1)\end{matrix}$where R is a resistance and X a reactance. The impedance may thereforebe written as:

$\begin{matrix}{Z = {\frac{1}{Y} = \frac{1}{G + {j\; B}}}} & (2)\end{matrix}$

Referring to FIG. 3B, there are illustrated four admittances Y₁, Y₂, Y₃,Y₄, where Y₁, Y₃ are seen by amplifier 41 and Y₂, Y₄ are admittancesseen by amplifier 42,

The admittance Y₃ seen by amplifier 41 and the admittance Y₄ seen byamplifier 42 may be expressed according to the following expressions:

$\begin{matrix}\left\{ \begin{matrix}{Y_{3} = {{\frac{2R_{L}}{Z_{0}^{2}} \cdot \frac{V_{L}}{V_{PEP}}}\left( {{\sin\;\varphi} + {j\;\cos\;\varphi}} \right)}} \\{Y_{4} = {{\frac{2R_{L}}{Z_{0}^{2}} \cdot \frac{V_{L}}{V_{PEP}}}\left( {{\sin\;\varphi} - {j\;\cos\;\varphi}} \right)}}\end{matrix} \right. & (3)\end{matrix}$

Where V_(PEP) is here considered to be a normalized maximum peak voltage(PEP—Peak Envelope Power), is the voltage over the load R_(L) 49. Z₀ isthe characteristic impedance of the quarter-wave transmission-line inthe combiner network and φ represents the outphasing angle of theChireix power amplifier system.

According to embodiments of the present invention, by placingshunt-reactances (−jX and +jX) as shown in FIG. 3A or FIG. 3B, one canachieve resonance with the reactive load seen by each amplifier 41, 42by altering e.g. φ. We express this as a susceptance B in the followingmanner:

$\begin{matrix}{B = {{\frac{2R_{L}}{Z_{0}^{2}} \cdot \frac{V_{L}}{V_{PEP}}}\sqrt{1 - \left( \frac{V_{L}}{V_{PEP}} \right)^{2}}}} & (4)\end{matrix}$

The normalized susceptance may be written as B′ and may be expressedaccording to the following equation:

$\begin{matrix}{B^{\prime} = {B \cdot \frac{Z_{0}^{2}}{2R_{L}}}} & (5)\end{matrix}$

The admittance Y₁ seen by amplifier 41 and the admittance Y₂ seen byamplifier 42 may further be expressed as a function of the susceptance Baccording to the following expression:

$\begin{matrix}\left\{ \begin{matrix}{Y_{1} = {Y_{3} - {j\; B}}} \\{Y_{2} = {Y_{4} + {j\; B}}}\end{matrix} \right. & (6)\end{matrix}$

For φ±0 Y₁ and Y₂ are entirely real as lm(Y₃)=+jB and lm(Y₄)=−jB. Thiswould not be the case if −jX and +jX are not present.

Using the above expressions of the admittances Y₁ and Y₂, we can,according to embodiments of the present invention, directly derive theexpressions of the impedances Z₁ and Z₂, where Z₁ represents theimpedance seen by amplifier 41 and Z₂ represents the impedance seen byamplifier 42. Z₁ and Z₂ are given according to the followingexpressions:

$\begin{matrix}\left\{ \begin{matrix}{Z_{1} = {\frac{1}{Y_{1}} = \frac{1}{Y_{3} - {j\; B}}}} \\{Z_{2} = {\frac{1}{Y_{2}} = \frac{1}{Y_{4} + {j\; B}}}}\end{matrix} \right. & (7)\end{matrix}$

From this, we can express +jX and −jX according to the following:

$\begin{matrix}{{j\; X} = {{j\;\omega\; L} = {\left. \frac{1}{j\;\omega\; C}\Rightarrow{j\; B} \right. = {\frac{1}{j\;\omega\; L} = {j\;\omega\; C}}}}} & (8)\end{matrix}$

Where ω=2πf_(c) being a function of the center frequency f_(c); L is aninductance and C is a capacitance.

It should be noted that we can use admittances Y₁ and Y₂ instead ofimpedances Z₁ and Z₂. In FIG. 3B, the admittances (or the impedances)seen by the amplifiers 41 and 42 are depicted. The voltage V_(L) overthe load R_(L) 49 is also shown.

It should also be noted that the input signal (not shown) to amplifier41 can be expressed as

${\frac{A}{2}{\cos\left( {{\omega\; t} - {\varphi(t)}} \right)}},$where A is the amplitude of the input signal, and the input signal (notshown) to amplifier 42 can similarly be expressed as

$\frac{A}{2}{{\cos\left( {{\omega\; t} + {\varphi(t)}} \right)}.}$These signals may, as previously described in conjunction to FIG. 2A, beproduced by a signal separator.

According to an embodiment of the present invention, the efficiency η ofthe composite amplifier 3 can be modeled using the following expression:

$\begin{matrix}{\eta = \frac{1}{\sqrt{1 + {\frac{1}{4}\left( \frac{{\sin\left( {2\varphi} \right)} - B_{c\;}^{\prime}}{\sin^{2}(\varphi)} \right)^{2}}}}} & (9)\end{matrix}$where B′_(C), as mentioned earlier, represents the normalizedsusceptance, which in this example is capacitive.

For a dynamic Chireix outphasing system in accordance to the presentinvention, the impedances seen by the power amplifiers, which are afunction of the tuneable reactances −jX and +jX may be dynamically tunedby dynamically tuning these reactances. The instantaneous efficiency ofthe composite amplifier is consequently increased.

The instantaneous efficiency (bottom figures) of the composite amplifierfor different tuning of the impedance(s)/admittances seen by eachamplifier and the corresponding Smith diagrams (top figures) areillustrated in FIGS. 4A-4D.

FIG. 4A illustrates an exemplary scenario when the peak efficiency isachieved as the outphasing angle φ increases at the points where the twoload trajectories cross each other over the real line (see the Smithdiagram). FIG. 4A also shows that by increasing φ, the output power isdecreased. The direction of increase of φ is illustrated by the arrowpointing towards the origin. In FIG. 4A there are also illustrated twonormalized output power levels where the instantaneous efficiencyreaches local maxima. The two efficiency peaks occur when both amplifierload impedance trajectories simultaneously cross each other over thereal line (horizontal line) in the Smith diagram.

FIG. 4B illustrates another exemplary scenario when the peak efficiencyis achieved as the outphasing angle φ further increases. In FIG. 4B,there are also illustrated two normalized output power levels where theinstantaneous efficiency reaches local maxima.

FIG. 4C illustrates yet another exemplary scenario of the instantaneousefficiency of a composite amplifier in accordance with an exemplaryembodiment of the present invention. As seen, the instantaneousefficiency is increased as the impedance seen by one of the amplifiersof the composite amplifier is dynamically tuned. It should be notedthat, in this exemplary embodiment, the impedance seen by an amplifieris a function of the reactance which in turn is a function of theoutphasing angle φ, and therefore the impedance may be dynamically tunedby changing φ.

If, according to the present invention, one continuously controls thereactances or similarly continuously controls the impedance/admittanceseen by each amplifier, the instantaneous efficiency of the compositeamplifier as a function of the output power backoff (in dB), willresemble the efficiency curve shown in FIG. 4D. The corresponding Smithdiagram is also illustrated.

From FIGS. 4A-4D we can deduct that the instantaneous efficiency of thecomposite amplifier is, according to embodiments of the presentinvention, increased when the impedance(s)/admittance(s) seen by theamplifiers are dynamically tuned. From FIGS. 4A-4D we can also concludethat the normalized output power level(s) where the instantaneousefficiency reaches local maxima is moved along with the envelopeamplitude fluctuations of the output signal and the instantaneousefficiency is, according to embodiments of the present invention,maximized over a longer time than that of the envelope amplitudefluctuations of the output signal.

Thus, by letting the point(s), corresponding to the normalized outputpower level(s) where the instantaneous efficiency reaches local maxima,move or track the envelope amplitude of the signal at slower speed thanthat of the envelope speed, the instantaneous efficiency of thecomposite amplifier is increased and we can still be able to haveamplitude headroom for quick upward excursions of the output signal. Thecomposite amplifier according to the present invention thus decouplesthe efficiency maximum from the amplitude maximum.

According to another embodiment of the present invention, if thereactances of a composite amplifier comprising a pair of amplifiers of aChireix outphasing system, are symmetrically retuned, the point(s),corresponding to the normalized output power level(s) where theinstantaneous efficiency reaches local maxima, can be moved withoutaltering the output signal of the composite amplifier. This scenario isillustrated in FIG. 5A where k represents a scaling constant that may beused to alter the impedance(s)/admittance(s) seen by each poweramplifiers. In FIG. 5A, the output voltage is illustrated as a functionof the scaling factor k for four different constant outphasing angles φ.Note that since the impedance(s)/admittance(s) seen by each poweramplifiers are a function of B (or B′) as presented in equations (6) and(7), the dynamic tuning/retuning of the impedances/admittances can beperformed by changing B (or B′). This is illustrated in FIG. 5B whichrepresents the instantaneous efficiency curves for different values ofB′. In FIG. 5B, the impedance(s) seen by each power amplifier of acomposite amplifier are, in accordance to embodiments of the presentinvention, dynamically tuned by altering B′ within the interval [0.3,1]. As can be observed in FIG. 5B, as the point(s) of high efficiencyi.e. the point(s) corresponding to the output power level(s) where theinstantaneous efficiency reaches local maxima is moved along with theenvelope amplitude of the output signal, the instantaneous efficiency isincreased. The instantaneous efficiency is thus arranged to be maximizedover a longer time than of the envelope amplitude fluctuations of theoutput signal. Tracking of the instantaneous efficiency is also shown inFIG. 5B (thick line).

Referring now to FIGS. 6A-6B, there are illustrated two differentstructures of a composite amplifier according to two exemplaryembodiments of the present invention. In both FIG. 6A and FIG. 6B, thecomposite amplifier 3 includes a pair of differently driven poweramplifiers 41, 42 of a Chireix outphasing system.

In FIG. 6A, which represents an example of a parallel implementation ofthe composite amplifier 3, the combiner network 53, in accordance withanother embodiment of the present invention, comprises a first varactornetwork 54 in parallel with a reactance 55 (an inductance) and a secondvaractor network 56 in parallel with another reactance 57 (acapacitance). Thus the impedance seen by amplifier 41 may be expressedas a function of the elements of the varactor network 54 and of thereactance 55. Similarly, the impedance seen by amplifier 42 may beexpressed as a function of the elements of the varactor network 56 andof the reactance 57. By dynamically tuning/retuning the above mentionedimpedances by e.g. dynamically tuning/retuning the varactor networks 54and 56, we can increase the instantaneous efficiency of the compositeamplifier 3. In other words, by tuning/retuning the varactor networks 54and 56, we can move the point(s) corresponding to the output powerlevel(s) where the instantaneous efficiency reaches local maxima. Theload impedance 49 is also illustrated. The element denoted RFC in FIG.6A represents DC (Direct Current) ground reference for the varactor(s)

In FIG. 6B, which represents an example of a series implementation ofthe composite amplifier 3, the combiner network 63, in accordance withanother embodiment of the present invention, comprises a first varactornetwork 64 placed in series with the reactance 65 (an inductance), and asecond varactor network 66 placed in series with the reactance 67 (acapacitance). Similarly to the parallel implementation described inconjunction with FIG. 6A, the instantaneous efficiency of the compositeamplifier 3 is also here increased by tuning/retuning theimpedance/admittance seen by amplifier 41 and the impedance/admittanceseen by amplifier 42. The tuning performed thus moves the point(s)corresponding to the output power level(s) where the instantaneousefficiency reaches local maxima. The load impedance 49 is alsoillustrated. The element denoted RFC in FIG. 6B represents DC (DirectCurrent) ground reference for the varactor(s)

It should be noted that the composite amplifier according to thepreviously described exemplary embodiments of the present invention, isnot restricted to the above illustrated implementations. As an example,the varactor network(s) may comprise a T-network comprising varactordiodes, a radio choke that may be arranged as an inductor such that thevaractor's intermediate frequency (IF) control signal is isolated from aRF signal. The varactor diode may also have a variable capacitance beinga function of a voltage impressed on terminals of the diode. Thevaractor network may also comprise a pair of Schottky diodes or aresistance having an appropriate resistance value e.g. within a range of1-5 Mohms.

Referring to FIG. 7A-7B, there are illustrated two different structuresof a composite amplifier according to other exemplary embodiments of thepresent invention. In both FIGS. 7A and 7B, the composite amplifiers 4,according to yet other exemplary embodiments of the present invention,include a pair of differently driven power amplifiers 71, 72 of aDoherty amplifier.

In FIG. 7A, the combiner network 73 of composite amplifier 4 comprisesdynamically tuneable reactances C1, L and C2. The composite amplifieralso comprises a load R_(Load) 49 connected to the combiner network andtwo differently driven power amplifiers 71 and 72 of which poweramplifier 71 represents the main amplifier of the Doherty amplifierwhereas amplifier 72 represents the peak amplifier of the Dohertyamplifier. The elements of the combiner network 73 i.e. C1, L and C2are, as illustrated in FIG. 7A, forming a pi-network. Thus, thequarter-wave transmission line of the composite amplifier 4 is hererepresented as a pi-network. In this network, the impedance seen by themain amplifier 71 is configured to be tuned/retuned by altering e.g. C1and L such that the instantaneous efficiency of the composite amplifier4 is increased. It should be noted that all three elements of thepi-network may be tuned/retuned. In addition, it is also possible todesign a pi-network as comprising of two inductances and onecapacitance. Furthermore, a T-network or a L-network may also be usedinstead of a pi-network. The present invention is therefore notrestricted to the structures presented in FIG. 7A and FIG. 7B.

In FIG. 7B another structure of a composite amplifier 4 is depicted. Asseen, the combiner network 83 comprises a circuit including shuntcapacitances C1, C2 and C3 and two quarter-wave transmission lines λ/4.In this exemplary embodiment of the present invention, the impedanceseen by the main amplifier 71 is configured to be tuned/retuned byaltering e.g. C1 and C2 such that the instantaneous efficiency of thecomposite amplifier 3 is increased. Again, all elements C1, C2 and C3may also be tuned or retuned. Thus by dynamically tuning the elements ofthe combiner network that connects the main amplifier to the commonoutput of the composite amplifier 4, we can make the efficiency maximumor the point(s) corresponding to the output power level(s) where theinstantaneous efficiency reaches local maxima, track the instantaneousenvelope amplitude of the output signal. The efficiency curves areillustrated in FIG. 8.

From FIG. 8 we can see that the instantaneous efficiency of thecomposite amplifier is “pointed” rather than “rounded” which indicatesthat the efficiency of a composite amplifier comprised of a Dohertyamplifier is less than that of a composite amplifier comprised of aChireix outphasing system. However, similarly to the previouslydescribed exemplary embodiment of the composite amplifier comprised of aChireix outphasing system, we can observe in FIG. 8 that, as thepoint(s) of high efficiency i.e. the point(s) corresponding to theoutput power level(s) where the instantaneous efficiency reaches localmaxima is moved along with the envelope amplitude of the output signal,the instantaneous efficiency is increased. The instantaneous efficiencyis thus also here arranged to be maximized over a longer time than ofthe envelope amplitude fluctuations of the output signal.

It should be noted that the composite amplifier comprising of a Dohertyamplifier and the composite amplifier comprising of a Chireix outphasingsystem, experience different sensitivity to certain losses and they aretherefore efficient for different situations. As an example andaccording to embodiments of the present invention, if substantial shuntlosses are present at the output of the amplifiers the impedance seen byone of the amplifiers may be seen as a resistance, being substantiallyinversely proportional to the square of the output amplitude's envelope,in parallel with (generally slightly) varying shunt loss resistance(plus some residual resistance). Below a certain amplitude value atwhich said resistance is equal to the shunt loss resistance, theimpedance(s) seen by one of the amplifiers is maintained fixed andlinear drive i.e. linear fundamental output current and voltage, isused. In a composite amplifier incorporating a Doherty amplifier, theamplitude value mentioned above may occur at a power that isapproximately R_(LOAD)/2R_(SHUNT) of the maximum output power for theentire composite amplifier, where R_(SHUNT) corresponds the shuntresistance of the composite amplifier. In a composite amplifierincorporating a Chireix outphasing system, the amplitude value mentionedabove may occur at a power that is approximately R_(LOAD)/R_(SHUNT) ofthe maximum output power for the entire composite amplifier.

In both cases above the impedance(s) seen by one of the amplifiers areconfigured to be tuned/retuned by e.g. changing a transformation ratioin the combiner network. According to embodiments of the presentinvention, the transformation ratio may be defined as the impedance seenat one end of the combiner network divided by the actual impedance atthe other end of the combiner network. The transformation ratio may e.g.be changed by tuning elements of the combiner network, for example, bytuning/retuning one or several capacitances or other tuneable reactancesof the combiner network.

Note that for a Doherty amplifier according to embodiments of thepresent invention, at the transition point, the voltage at the peakamplifier is substantially lower than that of the main amplifier, so theshunt loss of the peak amplifier is very small. The entire shunt loss atthis transition point is therefore approximately half of that of theChireix outphasing system, which gives the Doherty amplifier anadvantage in case of large shunt loss (low transistor shunt resistance).

For a Chireix outphasing system both amplifiers have largely the samevoltage which occurs at the amplitude value mentioned above. The lossfrom R_(SHUNT) is therefore largely double that of the Doherty and theamplitude value can, as mentioned above, be calculated as beingR_(LOAD)/R_(SHUNT) of the maximum output power i.e. the same as for asingle amplifier pure load modulation system.

We may therefore conclude that if shunt losses are relatively large, thecomposite amplifier comprised of a Doherty amplifier is considered moreefficient than that of a composite amplifier comprised of a Chireixoutphasing system. On the other hand, if no losses are present or iflosses are mainly dependent on the output radio frequency current, thecomposite amplifier incorporating a Chireix outphasing system isconsidered more efficient than that of a composite amplifierincorporating a Doherty amplifier. Nevertheless, the efficiency or theinstantaneous efficiency of the composite amplifiers is, according toembodiments of the invention, considered more efficient than that ofprior art composite amplifiers. This is because of the dynamic matching,as described before, is incorporated into a composite amplifier. Thedynamic matching is the dynamic tuning/retuning of the impedance(s) seenby each of the at least two differently driven power amplifiers of thecomposite amplifier. It should be noted that instead of dynamicallytuning impedances, transimpedances may instead be tuned.

Referring to FIG. 9, there is illustrated a radio terminal 100comprising a composite amplifier according to embodiments of the presentinvention. As shown in FIG. 9, the radio terminal 100 is provided with acomposite amplifier 120 in accordance with embodiments of the presentinvention. An antenna 130 representing a load impedance, is also shownconnected to the composite amplifier 120. In addition a general inputunit 110 for receiving input signal/signals such as modulated RF signalsis also depicted. Pre-processing operations on the input signal/signals,prior to forwarding it/them to the composite amplifier 130 are notdescribed here since these are not necessary for understanding thedifferent embodiments of the present invention. Furthermore, otherelements, which are not considered relevant for the understanding of thepresent invention, have been omitted.

The radio terminal 100 illustrated in FIG. 9 may be part of mobiletelephone: a radio base station or any other type of radio terminal thatis suitable for a wireless system. As an example, the radio terminal 100may be adapted for use in telecommunications wireless systems such asthe JDC (Japanese Digital Cellular), GSM (Global System for MobileCommunications), GPRS General Packet Radio Service), EDGE (Enhanced Datarates for GSM Evolution), WCDMA (Wide band Code Division MultiplexingAccess), CDMA (Code Division Multiplex Access), GPS (Global PositioningSystem), the WIMAX (Worldwide Interoperability for Microwave Access) orany other type of wireless system.

Although the present invention has been described with reference to acomposite amplifier incorporating a Doherty amplifier or a Chireixoutphasing system, it is evident that the invention is applicable tocomposite amplifiers with other type of amplifiers.

In addition, the present invention and its embodiments can be realisedin many ways. For example, one embodiment of the present inventionincludes a computer-readable medium having instructions stored thereonthat are executable by a computer system located in one or several radioterminals of a wireless system, for improving the instantaneousefficiency of a composite amplifier. The instructions executable by thecomputing system and stored on the computer-readable medium perform themethod steps of the present invention as set forth in the claims.

While the invention has been described in terms several embodiments, itis contemplated that alternatives, modifications, permutations andequivalents thereof will become apparent to those skilled in the artupon reading of the specifications and study of the drawings. It istherefore intended that the following appended claims include suchalternatives, modifications, permutations and equivalents as fall withinthe scope of the present invention.

1. A composite amplifier for use in a radio terminal of a telecommunications wireless system, comprising: at least two power amplifiers that are arranged to be connected to an output combiner network and to a load, the output combiner network comprising at least one dynamically tunable reactance; wherein said at least two power amplifiers are configured to be differently driven; wherein at least one of the impedances seen by each of said at least two power amplifiers is arranged to be dynamically tuned via tuning of the at least one dynamically tunable reactance such that the instantaneous efficiency of the composite amplifier is increased; and wherein said at least one of the impedances seen by each of said at least two differently driven power amplifiers appears to at least one of the two differently driven power amplifiers as a resistance that is substantially inversely proportional to the square of the output signal amplitude's envelope of said composite amplifier.
 2. The composite amplifier according to claim 1 wherein said at least one of the impedances seen by each of said at least two differently driven power amplifiers appears to at least one of the two differently driven power amplifiers as said resistance in parallel with a shunt loss resistance at the output of the composite amplifier.
 3. The composite amplifier according to claim 2 wherein said at least one of the impedances seen by each of said power amplifiers is arranged to be maintained fixed when the amplitude of the output signal from the composite amplifier is below a certain amplitude value at which said resistance is equal to said shunt loss resistance whereby linear drive of said composite amplifier is used.
 4. A method of increasing the instantaneous efficiency of a composite amplifier comprising at least two power amplifiers being connected to an output combiner network and to a load, said output combiner network comprising at least one dynamically tunable reactance, said method comprising: providing an input signal to each of the at least two power amplifiers of the composite amplifier, such that the at least two power amplifiers are differently driven; and dynamically tuning at least one of the impedances seen by each of said at least two power amplifiers via dynamic tuning of the at least one dynamically tunable reactance, such that the instantaneous efficiency of the composite amplifier is increased; wherein said at least one of the impedances seen by each of said at least two differently driven power amplifiers appears to at least one of the two power amplifiers as a resistance that is substantially inversely proportional to the square of the output signal amplitude's envelope of said composite amplifier.
 5. The method according to claim 4 wherein said at least one of the impedances seen by each of said at least two differently driven power amplifiers appears to said at least one of the power amplifiers as said resistance in parallel with a shunt loss resistance at the output of the composite amplifier.
 6. The method according to claim 5 further comprising maintaining said at least one of the impedances seen by each of said at least two differently driven power amplifiers fixed when the amplitude of the output signal from the composite amplifier is below a certain amplitude value at which said resistance is equal to said shunt loss resistance and further linearly driving said composite amplifier. 